Number Density is a term for the number of objects per unit volume.

    N
n = —
    V

for Number Density n, number of objects, N and volume V. The term is often used for atoms or molecules, e.g., in a gas, in contrast to other measures such as Mass Density.

Number Density can also be used for larger objects, e.g., stars.

A volume may have "partial" number densities, i.e., specific to one type of the objects in the volume. An alternative measure for some purposes is Partial Pressure, a calculation of the pressure contribution of one type of object, e.g., calculated by the ideal gas law. An example is Electron Pressure, which the Saha Equation can be adapted to use.